World Oceans Day — why is it plural?

There are five oceans:

• Atlantic Ocean
• Pacific Ocean
• Indian Ocean
• Arctic Ocean
• Southern (Antarctic) Ocean

and they are all considered on World Oceans Day.

This isn't the same construct as "World Bicycle Day" because that doesn't consider all the separate bicycles in the world, but rather the concept of cycling.

It would be "Всемирный день океанов" in Russian - we use the attributive in place of the genitive in English. The difference between the singular and plural comes across in Russian too - "Всемирный день океана" would have the same connotations as "World Ocean Day". That is to say ocean as a concept rather than all the oceans. Hope that helps.